Light modulation system



' g- 1 v v J. N. WHITAKYER 2,212,966,

LIGHT MODULATION SYSTEM Filed Oct. 2 1957 CARR/ER 5 Y FREQUENCY INPUTINVENTOR. 7 JAMES N. WHITAKER ATTORNEY.

Patented Aug. 27, 1940 2,212,966, LIGHT MODULATION SYSTEM James N.Whitaker, Tuckahoe, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application October 26, 1937, Serial No. 170,997

' IClaim. (01.1'79-1715) This invention relates to an electrical circuitto be used in'connection with a facsimile transmitter.

More specifically, the invention relates to a new and improved systemwhereby the signals produced by a light responsive device in thescanning head of a facsimile transmitting device may be modulated andamplified for subsequent transmission.

It is well known that photo-electric cells or other light responsiveelements may be used in facsimile transmitting systems to producesignals in accordance with conditions of light and shade to which suchcells are exposed, and in some instances the modulation of the carrierwave has been effected by means of a light chopper disc, together withthe use of associated circuit amplifiers and modulators. In suchsystems, however, wherein a light chopper is used, it has been 'foundthat the wave shape developed is not entirely satisfactory to provideadvantageous conditions of operation. Furthermore, the use of a lightchopper necessitates considerable mechanical apparatusand such apparatusis of course ine herently subject to erratic operation or failure.

Furthermore, in many of the facsimile scanners proposed heretofore, ithas been awkward and diflicult to place the photo-electrical cell in themechanical systems wherein light choppers are.

0 used without sacrificing certain desirable features. In the varioussystems using direct current amplifiers, difficulties have beenencountered due to a pronounced tendency of the amplifier to drift or todepart from its adjustment to such an extent that it materiallyinterferes with the w fidelity of the reproduction.

Another method which has previously been used is that of combining thephoto cell output 40 with a carrier tone by means of a mixing tube. Sucha system obviates the use of a mechanical chopper disc but introducescertain other disadvantages due to the fact that such systems aregenerally critical in adjustment and have a slight tendency to vary inresponse. Such a system wherein amixing tube is used is shown in thepatent to J. N. Whitaker #2,055,513, September 25, 1936.

so In the present invention the carrier frequency or oscillations arerrodulated in a new and improved manner with the result that nomechanically moving parts are used and with the further result that thesystem is more stable and pro- 55 duces signals having a more linearrelationship invention will become more apparent to those with respectto the light values to which the cell is exposed.

It is therefore one purpose of the present invention to produce anamplifying and modulating system for use in connection with a facsimile5 transmitting device wherein no mechanically moving parts are used, andwherein the modulation of the carrier frequency by the photo-cell outputis accomplished by entirely electrical means.

A further purpose of the present invention is the provision of anamplifying and modulating system for a facsimile transmitter wherein theoutput of the amplifier is reduced to a minimum when no light isdirected upon the photo-electric cell and wherein the output of thesystem increases in direct proportion to the amount of light which isdirected upon the photo-electric cell.

Further purposes and advantages of the present skilled in the art from areading of the following specification and claim, wherein:

'The single figure of the' drawing represents, for example, one form ofthe present invention.

Referring to the drawing a light responsive element or photo-electriccell 5 is provided upon which varying degrees of light are projected inaccordance with the light and dark shadings of the particular subjectmatter to be transmitted by facsimile. This photo-electric cell is, ofcourse, positioned in the scanning head of the facsimile transmitter,but for the purpose of clarity the system is shown schematically and thestructure of the scanning head is omitted. as

The source of carrier frequency is not shown, but the carrier current isapplied to the terminals l, which are connected to the primary oftransformer 2. The secondary of the transformer is shunted by aresistance 3, one terminal of which 40 is connected to ground. Anelectron discharge tube 4 is provided, which comprises at least theusual elements of an electron emitter, a control electrode and an anode.The control element of the'discharge tube is connected to a point along5 the resistance 3, in order that the carrier frequency as derived fromthe carrier oscillator will be impressed upon the tube 4 foramplification thereby and the amplified carrier frequency will, of.course, appear in the anode circuit of the tube. In order that'theanodemay be maintained at a proper potential with respect to theemitter, a source of positive potential is connected to the terminal 6.In this connection is included aresistance and the photo-electric cell5. At a u correspondsto the carrier frequency impressedupon the controlelctrode of the tube 4.

An amplifier tube I0 is provided and is coupled to the anode of tube 4by means of a condenser ii. The amplifier tube i0 is shown as a pentodein which the suppressor grid is connected internally to the emitterelement. The second grid, as well as the anode, are connected to thepositive source of potential. In the anode circuit of the amplifier tubeI0 is a transformer 68, the secondary Winding of which is connected tothe output terminals 20. In order that appropriate potentials may bemaintained on the emitter element and control electrode of the amplifiertube 60. a resistance i3 is provided, in parallel with which isconnected a condenser M. The resistance and condenser are connectedbetween the emitter and ground as shown in the drawing. To complete thecoupling of the amplifier tube It) to the output of the tube 3 anothercondenser, 52, is

wherein no mechanically moving parts are used tube s, howeve used. Agrid leak i6 is connected between the control electrode and ground asindicated in the drawing.

From the above description it may be seen that the conductivity ofdischarge tube 4 varies in accordance with the potential applied to thecontrol grid of that tube which is afiected solely by the application ofthe carrier frequency, but the mount of current which is conducted bythe' controlled by the resistance of the a de cir it, that is, theconductivity of the photo-elec ric cell 5. When little or no light isdirected on the photo-electric cell, the resistance of this element isrelatively high and as a result small variations in potentials willappear in the anode circuit of tube 4. As the light which is directeduponthe photo-electric cell is changed, then the corresponding change inthe resistance of the anode circuit causes changes in the amount ofcurrent which fiows in the anode circuit and as a result greatervariations in potentials will appear at the anode of the discharge tube4. These variations in potential are of course amplified by means of thedischarge tube 0 and finally appear at the output terminals 20 of thesystem.

From the above it may be seen that an amplifying andmodulating systemhas been devised and wherein amplitude modulation of a carrier frequencyis produced. Because of the fact that the light sensitive element 5 isincluded in the anode circuit of the discharge tube 4-, the potentialvariations that are impressed upon the amplifier tubev l0 bear a directand linear relationship to the light which iscaused to fall upon thephoto-electric cell 5' over the working range of the system.Furthermore, because of this circuit arrangement and its simplicity thesystem as a whole is not critical in adjustment and is inherently stablein its operation.

It is to'be understood that, although a simple three-element dischargetube isshown at 4, other discharge tubes may as well be used.Furthermore, it is not necessary that a pentode be used as an amplifiertube since other amplifier tubes may as well be used and also varyingdegrees of amplification may be employed depending upon the desiredresults.

It is also to be understood that, although the system is described asuseful particularly in connection with facsimile reproduction andtransmission, the system as shown and described may also be used inother respects wherein it is desired to modulate a carrier frequency inaccordance with changes in light values, the light values beingtranslated into electrical potential variations by any appropriatemeans.

Other modifications may be made in the present invention withoutdeparting from the spirit and scope thereof and it is desired that anyand all such modifications be considered within the purview of thepresent invention except as limited bythe art and the hereinafterappended claim.

Having now described my invention, what is claimed is:

A modulating system comprising a source of oscillations, an electrondischarge device including at least an electron emitter, a controlelectrode and an anode, a photoelectric tube, a circult coupling thecontrol electrode and the electron discharge device is controlled solelyin ac-' cordance with changes in impedance of the pho- 'toelectric tubeunder light activation thereof, an

amplifier tube including at least a cathode, a control electrode and ananode, and a capacity coupling means for coupling the control electrodeof said amplifier to one terminal of the photoelectric tube and a secondcapacity coupling means for coupling the cathode of said amplifier tothe other terminal of the photoelectric tube.

